That's a cool feature of Flickr...

Flickr takes the exif info embedded in an image (or perhaps info from your acount) and embeds it in the jpg in such a way that it survives transport and minor editing.

I try not to be nasty about it, and my copyright notices give fairly liberal use policy, but I do want credit.

So when I see one of my photos used without credit, I will often drop a "Umm, that's my photo, please credit me."